Voltage-controlled crystal oscillators are typically very accurate. Thus they are used in electronic circuits to produce, for example, precise clock signals. The frequency of the output signal of a crystal oscillator varies with changes in ambient temperature, however, and these changes affect the operation of the circuitry in which it is used. Thus it is desirable to control either the ambient temperature of the oscillator or the temperature-related variations in the oscillator frequency.
The ambient temperature of the oscillator may be controlled by enclosing it in an oven. One of the problems with using an oven is the relatively large amount of power required to operate it. It is therefore impractical to use ovens in circuits which are battery powered, such as cellular handheld telephone circuitry powered by an internal battery.
If controlling the ambient temperature of the oscillator is impractical, the frequency of the oscillator output signal may be maintained by compensating it, that is, altering the voltage which controls the oscillator to offset the temperature-driven frequency variations. The compensating voltage, or signal, must alter the oscillator output to precisely offset the change in the output signal due to temperature. Otherwise, the compensated oscillator output signal frequency is not accurate.
Temperature-driven frequency variations are not linearly related to changes in temperature. Thus accurate compensation is difficult. One compensation method employs a look-up table that contains compensating signal values corresponding to various temperatures. As the oscillator signal frequency varies due to changes in ambient temperature, compensation is accomplished by retrieving from the look-up table the compensating signal value associated with the temperature and applying a signal corresponding to the stored value to the oscillator as the voltage control signal.
The look-up table is generated by operating the oscillator at various temperatures and determining the control signal values which will return the oscillator output signal to the desired frequency. The size of the look-up table determines the accuracy of the compensation. A relatively large look-up table is required if the oscillator is operated over a wide temperature range and/or the tolerance for frequency variation is tight. Regardless of how large the look-up table is, errors are introduced when the ambient temperature is not precisely at a temperature for which a compensation value is stored.
Presumably, the stored value for the temperature closest to the ambient temperature is used as the compensating signal value. However, due to the non-linear relationship between temperature and frequency the error associated with using this compensating value may be relatively large for certain temperatures. Thus it is desirable to determine the compensating signal value corresponding to all temperatures within the operating range while at the same time using a look-up table which is of a manageable size.
An alternative to retrieving the compensating signal values from a look-up table is to generate them using a logical function generator. Ideally, the function generator is programmed with a function which mathematically represents the relationship between temperature and frequency, that is, the temperature-frequency transfer curve. The compensating signal values are calculated for various temperatures by applying to the logical function generator a signal, such as a thermistor signal, that varies monotonically with temperature. The generator then generates an associated compensating signal value by substituting the temperature signal value into the generator function.
In order to program the function generator, a function representing the entire temperature-frequency transfer curve must be found. If the transfer curve is irregular, which is the case for most crystal oscillators, a simple function will not suffice to describe it precisely. Thus the compensating signal values calculated by the function generator for certain portions of the transfer curve, that is, for certain temperatures within the temperature range, may not be accurate.
Crystal oscillators are also subject to variations in frequency due to the aging of the crystal. As the crystal ages the entire temperature-frequency transfer curve shifts up or down. Typically, to compensate for the effects of aging, a variable resistor is included in the circuit. The variable resistor is manually adjusted to alter the output signal frequency such that the shift is offset. Using this method of compensation, the oscillator circuitry must be physically accessible. In addition, the variable resistor must be a discrete component rather than part of an integrated circuit. These constraints add complexity to the circuit design and increase the cost of manufacture over that of a circuit which is fully integrated. Thus it is desirable to compensate for the aging of the crystal using integrated circuit technology.